Polishing method for semiconductor wafer and polishing apparatus for semiconductor wafer

ABSTRACT

A polishing method for a semiconductor wafer having a polishing target surface at a periphery portion thereof is disclosed. The method includes pressing a polishing member against the polishing target surface along a circumference of the semiconductor wafer by a plurality of pressing portions while rotating the semiconductor wafer in a circumferential direction, thereby polishing the polishing target surface of the semiconductor wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-308250, filed Nov. 14, 2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing method for a semiconductor wafer and a polishing apparatus for a semiconductor wafer.

2. Description of the Related Art

With miniaturization of a semiconductor device, polishing a rim portion (periphery portion) of a semiconductor wafer is becoming important (see, e.g., JP-A 2005-7518 (KOKAI)). Usually, a polishing member, e.g., a polishing tape is pressed against a polishing target surface at a periphery portion of a wafer to perform polishing while rotating the wafer in a circumferential direction.

However, since the periphery portion of the wafer has a curvature in the circumferential direction, pressing the polishing member against the polishing target surface with a uniform pressure is difficult. That is, the pressure is relatively increased at a central part of a polishing region, but the pressure is relatively decreased at both ends of the polishing region. Therefore, a large pressure difference occurs between the central part and both the ends of the polishing region.

In a conventional technology, when pressing the polishing member against the polishing target surface at the periphery portion of the semiconductor wafer in this manner, pressing the region that is wide in the circumferential direction with the uniform pressure is difficult. Therefore, effectively applying the pressure to the wide region at the periphery portion of the semiconductor wafer to carry out polishing is difficult.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a polishing method for a semiconductor wafer having a polishing target surface at a periphery portion thereof, the method comprising: pressing a polishing member against the polishing target surface along a circumference of the semiconductor wafer by a plurality of pressing portions while rotating the semiconductor wafer in a circumferential direction, thereby polishing the polishing target surface of the semiconductor wafer.

According to a second aspect of the present invention, there is provided a polishing apparatus for a semiconductor wafer having a polishing target surface at a periphery portion thereof, comprising: a rotation unit which rotates the semiconductor wafer in a circumferential direction; and a pressing unit which presses a polishing member against the polishing target surface of the semiconductor wafer which is rotated by the rotation unit to polish the polishing target surface, the pressing unit having a plurality of pressing portions which press the polishing member against the polishing target surface along a circumference of the semiconductor wafer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a view schematically showing an outline structure of a polishing apparatus for a semiconductor wafer according to a first embodiment of the present invention;

FIG. 2 is a view schematically showing an outline structure of the polishing apparatus for the semiconductor wafer according to the first embodiment of the present invention;

FIG. 3 is a view showing a distribution of a pressure applied to a polishing target surface according to the first embodiment of the present invention;

FIGS. 4A and 4B are views schematically showing a positional relationship between the polishing target surface and a polishing tape according to the first embodiment of the present invention;

FIG. 5 is a view schematically showing an outline structure of a polishing apparatus for a semiconductor wafer according to a second embodiment of the present invention;

FIG. 6 is a view schematically showing an outline structure of a polishing apparatus for a semiconductor wafer according to a third embodiment of the present invention;

FIG. 7 is a view for explaining a polishing method for a semiconductor wafer according to a fourth embodiment of the present invention;

FIG. 8 is a view showing a polishing region according to the fourth embodiment of the present invention; and

FIG. 9 is a view showing a polishing region according to the fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments according to the present invention will now be explained hereinafter with reference to the accompanying drawings.

Embodiment 1

FIGS. 1 and 2 are views schematically showing an outline structure of a polishing apparatus for a semiconductor wafer according to a first embodiment of the present invention. FIG. 1 is a view seen from a direction parallel to a main surface of a semiconductor wafer, and FIG. 2 is a view seen from a direction vertical to the main surface of the semiconductor wafer.

This polishing apparatus includes a rotation unit 20 which rotates a semiconductor wafer 10 in a circumferential direction, and a pressing unit 30 which presses a polishing tape (a polishing member) 41 against a polishing target surface to polish the polishing target surface at a rim portion (periphery portion) 11 of the semiconductor wafer 10 that is rotated by the rotation unit 20. Further, this polishing apparatus includes a tape movement unit 50 that moves the polishing tape 41, and the tape movement unit 50 is formed of a tape supplying reel 51 and a tape collecting reel 52.

The rotation unit 20 has a holding portion 21 that holds the semiconductor wafer 10 in a vacuum chuck system, and a rotation shaft 22 that is used to rotate the holding portion 21. When the semiconductor wafer 10 held by the holding portion 21 is rotated around the rotation shaft 22, the polishing target surface at the periphery portion 11 of the semiconductor wafer 10 can be polished by the polishing tape 41.

The pressing unit 30 has an elastic member 31 formed of a silicone rubber, a plurality of pressing heads (displacement portions) 32 serving as a plurality of pressing portions, a plurality of shafts 33 which displace the pressing heads 32, a plurality of cylinders 34 which hold the shafts 33, and a controller 35 which controls displacements of the pressing heads 32. The pressing heads 32 can move in a vertical direction to a contact region of the polishing tape 41 and the polishing target surface of the semiconductor wafer 10. When the pressing heads 32 are displaced, the elastic member 31 and the polishing tape 41 are deformed. As a result, the polishing tape 41 can be pressed against the polishing target surface at the periphery portion 11 of the semiconductor wafer 10 along a circumference of the semiconductor wafer 10. Displacements of the respective pressing heads 32 can be independently controlled by the controller 35.

FIG. 3 is a view showing a distribution of a pressure applied to the polishing target surface in the circumferential direction when the polishing tape 41 is pressed against the polishing target surface of the semiconductor wafer 10 by the pressing unit 30. In this embodiment, the pressing unit 30 has the three pressing heads 32, and hence the pressure distribution has three peaks based on displacements of the pressing heads 32. That is, pressing forces of the plurality of pressing portions (the pressing heads 32) enable providing the plurality of peaks in the pressure distribution. Since the peaks in the pressure distribution can be dispersed in this manner according to this embodiment, the polishing tape 41 can be pressed against the polishing target surface at the periphery portion 11 of the semiconductor wafer with the pressure that is substantially uniformed in a wide region. Therefore, the pressure can be effectively applied to the wide region in the polishing target surface to perform polishing. As a result, a polishing efficiency can be improved to shorten a polishing time.

Furthermore, according to this embodiment, displacements of the respective pressing heads 32 can be independently controlled. Therefore, accurately controlling displacement amounts of the respective pressing heads 32 enables optimizing the pressure distribution. For example, heights of the plurality of peaks (intensities of the pressure) in the pressure distribution can be uniformed. Therefore, the above-explained effect can be further enhanced.

FIGS. 4A and 4B are views schematically showing a positional relationship between the polishing target surface of the semiconductor wafer 10 and the polishing tape 41. FIG. 4A is a view showing a position of the polishing tape 41 when polishing an end facet (the polishing target surface) at the periphery portion 11 of the semiconductor wafer 10, and FIG. 4B is a view showing a position of the polishing tape 41 when polishing a slant surface (the polishing target surface) 13 at the periphery portion 11 of the semiconductor wafer 10.

A curvature radius of the end facet 12 is smaller than that of the slant surface 13. Therefore, when a conventional polishing apparatus is used to perform polishing, a polishing region width of the end facet 12 becomes narrower than that of the slant surface 13. Accordingly, when polishing the end facet 12, a region that is not used for polishing is relatively increased in a width direction of the polishing tape 41. As a result, an amount of using the polishing tape 41 is necessarily increased, which results in a rise of a polishing cost.

In this embodiment, the pressing unit 30 has the plurality of pressing heads 32 as explained above. Therefore, the polishing tape 41 can be pressed against the end facet 12 over the wide region when polishing the end facet 12. Accordingly, a width of the region used for polishing can be increased, and an amount of using the polishing tape 41 can be reduced.

An operation of this embodiment will now be explained.

First, as shown in FIG. 1, the semiconductor wafer 10 is set on the holding portion 21 of the rotation unit 20. Moreover, the semiconductor wafer 10 is rotated in the circumferential direction, and the polishing tape 41 is pressed against the polishing target surface at the periphery portion 11 of the semiconductor wafer 10 by using the plurality of pressing heads 32 to polish the polishing target surface. Additionally, polishing is carried out while supplying the polishing tape 41 from the tape supplying reel 51 to the tape collecting reel 52. Based on this polishing, a film (e.g., a silicon oxide film or a silicon nitride film) that has adhered to the end facet 12 or the slant surface 13 of the semiconductor wafer 10 can be removed.

A rotating speed of the semiconductor wafer 10 is, e.g., 500 rpm. Further, as the polishing tape 41, one obtained by, e.g., securing diamond abrasives on a polyethylene terephthalate (PET) base material (e.g., a width: 70 mm, a thickness: 50 μm) by a binder is used. A feed speed of the polishing tape 41 is, e.g., 50 mm/min.

Although a polishing order of the end facet and the slant surface is not restricted in particular, polishing is carried out in the order of, e.g., the upper slant surface, the end facet, and the lower slant surface. When polishing each surface, a displacement amount of each pressing head 32 is accurately controlled in accordance with a curvature of each surface so that an optimum pressure can be applied in the width direction of the polishing tape 41. As a result, both the end facet and the slant surface can be polished with the pressure optimized in the wide region.

As explained above, according to this embodiment, since the pressing unit has the plurality of pressing portions, the plurality of peaks can be provided in the pressure distribution in the circumferential direction of the semiconductor wafer. Since the peaks in the pressure distribution can be dispersed in this manner, the pressure substantially uniformed in the wide region can be applied. Therefore, since the pressure can be effectively applied to the wide region at the periphery portion of the semiconductor wafer to perform polishing, the polishing efficiency can be improved, and the polishing time can be reduced. Furthermore, a width of the polishing tape use region can be increased, and an amount of using the polishing tape can be decreased. Moreover, in this embodiment, displacements of the plurality of pressing portions are independently controlled. Therefore, the pressure distributions can be individually optimized with respect to the respective surfaces (the polishing target surfaces) with different curvatures at the periphery portion of the semiconductor wafer. Additionally, each polishing target surface can be polished in the thus optimized pressure distribution.

It is to be noted that the polishing tape is used as the polishing member in this embodiment, such a fixed abrasive pad as explained in the following third embodiment may be used as the polishing member.

Embodiment 2

A second embodiment according to the present invention will now be explained. Since a basic structure of a polishing apparatus and a basic polishing method are the same as those in the first embodiment, thereby omitting an explanation of the particulars described in the first embodiment.

FIG. 5 is a view schematically showing an outline structure of a polishing apparatus according to this embodiment.

In this embodiment, a structure of a pressing unit 30 is different from that in the first embodiment. In the pressing unit 30 according to this embodiment, an elastic member 31 is formed of a base portion 31 a and a plurality of convex portions 31 b, and the plurality of convex portions 31 b function as a plurality of pressing portions. Specifically, the convex portions 31 b are fixed to the base portion 31 a positioned between the convex portions 31 b and a polishing tape (a polishing member) 41. Further, although the plurality of pressing heads 32, the plurality of shafts 33, and the plurality of cylinders 34 are provided in the first embodiment, the number of a pressing head 32, the number of a shaft 33, and the number of a cylinder 34 are all one in this embodiment.

As explained above, the plurality of convex portions 31 b are interposed between the pressing head 32 and the polishing tape 41 in this embodiment. Therefore, when the pressing head 32 is displaced, the convex portions 31 b are pressed, and pressing forces from the convex portions 31 b enable pressing the polishing tape 41 against a polishing target surface at a periphery portion 11 of a semiconductor wafer 10.

When the polishing tape 41 is pressed against the periphery portion of the semiconductor wafer 10, a displacement amount of the central convex portion 31 b becomes larger than those of the convex portions 31 b provided on both sides. Therefore, increasing heights of the convex portions 31 b provided on both sides to be larger than a height of the central convex portion 31 b is desirable to enable accurately pressing all the three convex portions 31 b by using the pressing head 32. Alternatively, sizes of the convex portions 31 b on both sides may be increased to be larger than a size of the central convex portion 31 b. Alternatively, hardness of the convex portions 31 b on both sides may be increased beyond that of the central convex portion 31 b. That is, it is preferable that at least one of the height, the size, and the hardness of each convex portion 31 b is adjusted in accordance with an arrangement position of each convex portion 31 b. Furthermore, as explained in the first embodiment, a curvature radius of an end facet of the semiconductor wafer 10 is different from that of a slant surface of the same. Therefore, it is desirable for a distal end of each convex portion 31 b to have a rounded shape in order to enable accurately pressing all the three convex portions 31 b by using the pressing head 32 irrespective of the curvature radius.

As explained above, in this embodiment, the pressing unit 30 has three convex portions 31 b. Therefore, a pressure distribution has three peaks corresponding to the three convex portions 31 b. That is, pressure forces of the plurality of pressing portions (the convex portions 31 b) enable providing the plurality of peaks in the pressure distribution in a circumferential direction of the semiconductor wafer. Therefore, like the first embodiment, the peaks in the pressure distribution can be dispersed in this embodiment. Consequently, the polishing tape 41 can be pressed against the periphery portion 11 of the semiconductor wafer 10 with a pressure substantially uniformed over a wide region. Therefore, the pressure can be effectively applied to the wide region to polish both the end facet and the slant surface of the semiconductor wafer 10. Moreover, adjusting the heights, the sizes, or the hardness of the plurality of convex portions 31 b enables optimizing the pressure distribution. As a result, like the first embodiment, a polishing efficiency can be improved, and a polishing time can be shortened. Additionally, like the first embodiment, a width of a polishing tape use region can be increased, thereby decreasing an amount of using the polishing tape.

It is to be noted that the polishing tape is used as the polishing member in this embodiment, such a fixed abrasive pad as explained in the following third embodiment may be used as the polishing member.

Embodiment 3

A third embodiment according to the present invention will now be explained. It is to be noted that a basic structure of a polishing apparatus and a basic polishing method are similar to those in the first embodiment, thereby omitting an explanation of the particulars described in the first embodiment.

FIG. 6 is a view schematically showing an outline structure of a polishing apparatus according to this embodiment.

In this embodiment, a structure of a pressing unit 30 is different from that in the first embodiment. The pressing unit 30 according to this embodiment has a plurality of hollow portions (displacement portions) 36 which are formed of membranes and function as a plurality of pressing portions, a plurality of tubes 37 connected with the hollow portions 36, a gas supply portion 38 which supplies a gas, e.g., air into the hollow portions 36 via the tubes 37, and a controller 35 which controls displacements of the hollow portions 36. Internal pressures of the respective hollow portions 36 can be independently controlled by the controller 35.

Further, in this embodiment, a fixed abrasive pad 42 is used as a polishing member. The fixed abrasive pad 42 does not move like the polishing tape but is fixed at a predetermined position. This fixed abrasive pad 42 is obtained by, e.g., burying ceria abrasives in a resin having elasticity.

In this embodiment, when an amount of supplying the gas into the hollow portions 36 is controlled, displacements of the respective hollow portions 36 can be controlled. As a result, the fixed abrasive pad 42 is deformed to enable pressing the fixed abrasive pad 42 against a periphery portion of a semiconductor wafer 10.

Furthermore, in this embodiment, the internal pressures of the respective hollow portions 36 can be independently controlled. Therefore, accurately controlling the internal pressures of the respective hollow portions 36 enables precisely controlling displacements of the respective hollow portions 36, thereby optimizing a pressure distribution. It is to be noted that a relatively large force must be applied to a peripheral part rather than a central part of the fixed abrasive pad 42 in order to deform the fixed abrasive pad 42 along the periphery portion of the semiconductor wafer 10. Therefore, it is desirable to increase the internal pressures of the hollow portions 36 from the hollow portion 36 at the central part toward the hollow portions 36 at the peripheral part.

As explained above, in this embodiment, since the pressing unit 30 has the plurality of hollow portions 36, a plurality of peaks can be provided in a pressure distribution in a circumferential direction of the semiconductor wafer 10 by pressing forces from the plurality of pressing portions (the hollow portions 36). Therefore, like the first embodiment, peaks in the pressure distribution can be dispersed in this embodiment. Therefore, the fixed abrasive pad 42 can be pressed against a polishing target surface at the periphery portion 11 of the semiconductor wafer 10 with a pressure substantially uniformed in a wide region. Further, in this embodiment, the internal pressures of the respective hollow portions 36 are independently controlled. Therefore, the pressure distributions can be individually optimized with respect to respective surfaces (polishing target surfaces) with different curvatures at the periphery portion 11 of the semiconductor wafer 10. Furthermore, the polishing target surfaces can be polished in the thus optimized pressure distributions. Therefore, like the first embodiment, the pressure can be effectively applied to the wide region to polish both an end facet and a slant surface of the semiconductor wafer 10. As a result, like the first embodiment, a polishing efficiency can be improved, and a polishing time can be reduced.

It is to be noted that the fixed abrasive pad is used as the polishing member in this embodiment, but the polishing tape can be used as the polishing member like the first and second embodiments.

Embodiment 4

A fourth embodiment according to the present invention will now be explained. It is to be noted that a basic structure of a polishing apparatus and a basic polishing method are the same as those in the first embodiment, thereby omitting an explanation of the particulars described in the first embodiment.

FIG. 7 is a view for explaining a polishing method according to this embodiment.

In this embodiment, a polishing target surface at a periphery portion of a semiconductor wafer 10 is polished while moving a polishing region of a polishing tape (a polishing member) 41 in a circumferential direction of the semiconductor wafer 10. For example, as shown in FIG. 7, polishing is carried out while gradually moving a polishing head 60 in the circumferential direction of the semiconductor wafer 10. It is to be noted that the elastic member 31, the pressing heads 32, the shafts 33, and the cylinders 34 in the polishing apparatus according to the first embodiment substantially correspond to the polishing head 60, for example. It is assumed that a movement angle θ (a maximum movement angle from a movement center) of the polishing head 60 is approximately, e.g., 3° and a movement speed of the same is approximately 0.3°/second. Since the polishing head 60 is moved in the circumferential direction of the semiconductor wafer 10 in this manner, a region of the polishing tape 41 which is pressed by the polishing head 60 moves, and a polishing region of the polishing tape 41 (a range of a region used for polishing) moves.

FIG. 8 is a view showing a polishing region 45 of the polishing tape 41. Since the polishing head 60 moves in a vertical direction to a movement direction of the polishing tape 41 (a tape feed direction), the polishing region 45 moves in an oblique direction.

As explained in the first embodiment, a width of a polishing region of an end facet of the semiconductor wafer 10 is generally narrower than a width of a polishing region of a slant surface of the same. Therefore, when polishing the end facet, increasing the movement angle θ of the polishing head 60 beyond the counterpart when polishing the slant surface is desirable. As a result, when polishing the end facet, the polishing tape 41 can be effectively used in a width direction of the polishing tape 41. Therefore, when polishing the end facet, the polishing tape 41 can be used in a wide region, thereby decreasing an amount of using the polishing tape 41.

It is to be noted that polishing can be carried out while oscillating the polishing head 60. For example, when polishing the end facet of the semiconductor wafer 10, the movement angle (an oscillation angle) θ is set to approximately 3°, and an oscillation speed is set to approximately 100 rpm. When polishing the slant surface of the semiconductor wafer 10, the movement angle (the oscillation angle) θ is set to approximately 0.5°, and the oscillation speed is set to approximately 150 rpm. In this case, the polishing tape 41 can be used in a wide region, thus reducing an amount of using the polishing tape 41.

Moreover, such a fixed abrasive pad as described in the third embodiment may be used in place of the polishing tape. In this case, the fixed abrasive pad is reciprocated in the circumferential direction of the semiconductor wafer 10, and the fixed abrasive pad is moved in a vertical direction to the circumferential direction of the semiconductor wafer 10. Consequently, as shown in FIG. 9, a polishing region (a range of a region used for polishing) 45 of the fixed abrasive pad 42 moves in a zigzag pattern. Therefore, the fixed abrasive pad 42 can be used in a wide region.

It is to be noted that the method explained in this embodiment can be appropriately applied to the polishing apparatuses and the polishing methods described in the first to third embodiments. Therefore, in this embodiment, the same effect as the basic effect explained in the first to third embodiments can be obtained.

Additionally, since polishing is performed while moving the polishing region of the polishing member in the circumferential direction of the semiconductor wafer in this embodiment, the polishing member can be used in a wide region. Further, when the polishing head or the polishing member is oscillated, the peak positions in the pressure distribution explained in the first to third embodiments move with oscillation, and hence the peak positions in the pressure distribution can be averaged in terms of time. Accordingly, a pressure can be prevented from being concentrated on a specific position of the polishing member, thereby averaging the pressure applied to the polishing member.

Incidentally, in each of the foregoing embodiments, as a material of a polishing target film, there are, e.g., SiO₂, SiN, Si, poly-Si, a-Si, Hf, Al₂O₃, Ta, Ti, Ru, Al, W, Cu, Ni, NiSi_(x), Co, CoSi_(x), and others.

Furthermore, as a base material of the polishing tape, there are a resin film (e.g., PET or PPS) a nonwoven fabric, a fabric, and others. As the abrasive, there are, e.g., diamond, SiC, Al₂O₃, SiO₂, Ceria, TiO₂, and others.

Moreover, as a polishing type, there is polishing which is of a free abrasive type (polishing using a slurry and a polishing pad) as well as polishing which is of a fixed abrasive type (polishing using the polishing tape or the fixed abrasive pad).

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A polishing method for a semiconductor wafer having a polishing target surface at a periphery portion thereof, the method comprising: pressing a polishing member against the polishing target surface along a circumference of the semiconductor wafer by a plurality of pressing portions while rotating the semiconductor wafer in a circumferential direction, thereby polishing the polishing target surface of the semiconductor wafer.
 2. The method according to claim 1, wherein the plurality of pressing portions include a plurality of individually controllable displacement portions.
 3. The method according to claim 2, wherein each of the displacement portions has a pressing head which is movable in a direction substantially vertical to a contact region of the polishing member and the polishing target surface.
 4. The method according to claim 3, wherein an elastic member is interposed between the pressing heads and the polishing member.
 5. The method according to claim 2, wherein each of the displacement portions has a hollow portion whose internal pressure is controllable.
 6. The method according to claim 2, wherein a displacement amount of each of the displacement portions is controlled in accordance with a curvature of the polishing target surface.
 7. The method according to claim 1, wherein the plurality of pressing portions include a plurality of convex portions.
 8. The method according to claim 7, wherein the convex portions have elasticity.
 9. The method according to claim 8, wherein the plurality of convex portions are fixed to a base portion that is positioned between the convex portions and the polishing member and has elasticity.
 10. The method according to claim 7, wherein at least one of a height, a size, and hardness of each of the convex portions is adjusted in accordance with an arrangement position of each of the convex portions.
 11. The method according to claim 7, wherein a distal end of each of the convex portions has a rounded shape.
 12. The method according to claim 1, wherein a distribution of a pressure applied to the polishing target surface has a plurality of peaks based on the plurality of pressing portions.
 13. The method according to claim 12, wherein heights of the plurality of peaks are substantially equal to each other.
 14. The method according to claim 1, wherein the polishing target surface is polished while moving a polishing region of the polishing member in the circumferential direction of the semiconductor wafer.
 15. The method according to claim 1, wherein the polishing member is selected from a polishing tape and an abrasive pad.
 16. A polishing apparatus for a semiconductor wafer having a polishing target surface at a periphery portion thereof, comprising: a rotation unit which rotates the semiconductor wafer in a circumferential direction; and a pressing unit which presses a polishing member against the polishing target surface of the semiconductor wafer which is rotated by the rotation unit to polish the polishing target surface, the pressing unit having a plurality of pressing portions which press the polishing member against the polishing target surface along a circumference of the semiconductor wafer.
 17. The apparatus according to claim 16, wherein the plurality of pressing portions include a plurality of individually controllable displacement portions.
 18. The apparatus according to claim 17, wherein each of the displacement portions has a pressing head which is movable in a direction substantially vertical to a contact region of the polishing member and the polishing target surface.
 19. The apparatus according to claim 17, wherein each of the displacement portions has a hollow portion whose internal pressure is controllable.
 20. The apparatus according to claim 16, wherein the plurality of pressing portions include a plurality of convex portions. 